Fuel cells include three components: a cathode, an anode, and an electrolyte that is sandwiched between the cathode and the anode and passes only protons. Each electrode is typically coated on one side by a catalyst. In operation, the catalyst on the anode splits hydrogen into electrons and protons. The electrons are distributed as electric current from the anode, through an electrical load (e.g., a drive motor) and then to the cathode, where as the protons migrate from the anode, through the electrolyte to the cathode. The catalyst on the cathode combines the protons with electrons returning from the electrical load (e.g., a drive motor) and oxygen from the air to form water. Individual fuel cells can be stacked together in a series to generate increasing larger quantities (i.e., voltages) of electricity.
In a Polymer-Electrolyte-Membrane (PEM) fuel cell, a polymer electrode membrane serves as the electrolyte between a cathode and an anode. The polymer electrode membrane currently being used in fuel cell applications requires a certain level of humidity to facilitate proton conductivity. Therefore, maintaining the proper level of humidity in the membrane, through humidity-water management, is desirable for proper (or optimal) function and extended durability (or life) of the fuel cell.
In order to prevent leakage of the hydrogen gas and oxygen gas supplied to the electrodes and prevent mixing of the gases, a gas sealing material and gaskets are arranged on the periphery of the electrodes, with the polymer electrolyte membrane sandwiched therebetween. The sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane to form a membrane electrode assembly (MEA). Disposed outside of the MEA, are conductive separator plates for mechanically securing the MEA and electrically connecting adjacent MEAs in series. A portion of the separator plate, which is disposed in contact with the MEA, is provided with a gas passage for supplying hydrogen or oxygen fuel gas to the electrode surface and removing generated water.
The presence of liquid water in automotive fuel cells is typically common because appreciable quantities of water are generated as a by-product of the electro-chemical reactions during fuel cell operation. Furthermore, saturation of the fuel cell membranes with water can result from rapid changes in temperature, relative humidity, and operating and shutdown conditions. Excessive membrane hydration may result in flooding, excessive swelling of the membranes and the formation of differential pressure gradients across the fuel cell stack.
Cell performance is influenced by the formation of liquid water or by dehydration of the ionic exchange membrane. Water management and the reactant distribution have a major impact on the performance and durability of fuel cells. Cell degradation with mass transport losses due to poor water management still remains a concern for automotive and other applications. Long exposure of the membrane to liquid water can also cause irreversible material degradation. Water management strategies such as pressure drop (e.g., gas flow velocity and/or pressure drop across the membrane), temperature gradients and counter flow operations have been implemented and been found to improve mass transport to some extent especially at high current densities. Good water management, however, is still needed for optimal performance and durability of a fuel cell stack.